1. Field of the Invention
The invention relates generally to the field of measuring the registration of overlaid patterns by the use of electromagnetic radiation and, more particularly, to using the radiation reflected from different portions of overlaid patterns to measure any error in the registration of the overlaid patterns.
2. Description of the Prior Art
There have been proposed in the prior art many techniques for optically measuring the registration or alignment of two planar surfaces, such as a photomask and a semiconductor wafer.
For example, Kleinknecht et al in U.S. Pat. No. 4,211,489 have disclosed an automatic photomask alignment system in which a monochromatic light beam is directed through a key on a photomask onto a diffraction grating on a semiconductor substrate to produce a pattern of light spots whose intensities at various locations are determined by the relative alignment of the photomask and substrate.
Fay et al in U.S. Pat. No. 4,311,389 have disclosed a method of aligning a mask plane with a pattern plane. Linear Fresnel zones are inscribed on the mask plane, and a reflecting grating is inscribed on the pattern plane. A monochromatic light beam is directed through the Fresnel zones onto the reflecting grating such that the intensity of the reflected light is an indication of the alignment of the two planes.
Wang et al in U.S. Pat. No. 4,327,292 have disclosed an alignment process in which a focused laser beam is directed onto alignment marks on first and second surfaces to be aligned. The reflected beams are detected to form respective first and second serial signals which are compared in phase with a reference signal to produce respective error signals indicative of the misalignment of the two surfaces.
Westell in U.S. Pat. No. 3,867,038 has disclosed a system for optically aligning two objects by matching images of fixed reticles on the objects to a rotating reticle by convolution of the fixed reticle images with the rotating reticle.
Other registration and alignment measuring techniques are disclosed in U.S. Pat. Nos. 3,488,106; 3,531,196; 3,660,157 and 4,009,453; and in the following IBM Technical Disclosure Bulletins; Vol. 16, No. 1, June 1973, page 16; Vol. 18, No. 4, September 1975, pages 1110-1112; Vol. 19, No. 4, September 1976, page 1224; Vol. 19, No. 4, September 1976, page 1459; Vol. 22, No. 3, August 1979, pages 1054-1055; Vol. 22, No. 10, March 1980, pages 4440-4441; Vol. 22, No. 12, May 1980, page 5475; Vol. 23, No. 4, September 1980, page 1366; and Vol. 23, No. 7A, December 1980, pages 2996-2998.